Ruby on Rails, a rapid-fire Web development stack, has been under fire of late. Twitter outages have led a fairly loud chorus of folks singing the "Ruby doesn't scale" song.

But is it really fair to make the Ruby-Twitter connection, which by the way was hit with another outage (Techmeme). In a blog post, Twitter said:

Downtime is not good. We caused a database to fail during a routine update early this afternoon. We switched to a replica and expect this recovery to take place quickly. We're all working on it and watching right now as Twitter gets back up to speed. We have a thread open on our support forum which we'll update when we have more details to share. Getting our act together is something we continue to work on as we grow our company and our service.

I spoke to Ezra Zygmuntowicz (right), co-founder and director of software engineering at Engine Yard, about Ruby on Rails and the Twitter connection. Zygmuntowicz, who built one of the first production Rails applications at the Yakima Herald in 2004, was plugging his recently published book: "Deploying Rails Applications: A Step-by-Step guide." The book was rewritten two or three times (it's hard to write a book about a platform where deployment techniques change every six to eight months).

Here are some highlights from our conversation:

On whether Ruby on Rails can scale:

Zygmuntowicz was adamant that Ruby on Rails, which speeds up time to market because it has a stack template, controllers, a model layer with database connections. I've interviewed dozens of folks asking them how do you scale and the back and forth on Ruby was telling. There seems to be little middle ground on the issue. Zygmuntowicz had a simple take: "Languages don't scale. Architectures scale." "I'm of the opinion is that Ruby does scale," he said, adding that Ruby isn't any different than PHP or a Java deployment.

Zygmuntowicz acknowledged that Twitter is the "poster child for Rails" and its outages have stoked the idea that Ruby on Rails can't scale. However, Zygmuntowicz noted that many scaling problems intersect with database issues, which echoes comments from other scalability experts I've talked to. "Twitter's problem has nothing to do with Rails. It has mostly to do with the size of the social graph and the way they've done their database," said Zygmuntowicz. "Languages don't have scalability built in." Translation: Twitter allows folks to have unlimited friends. So the friend of a friend go round taxes that databases because every message, follower, etc. is a field update.

"It is kind of unfortunate that everybody blames Twitter's problems on Ruby on Rails. Ninety percent of Twitter traffic is API calls, not Web traffic. Twitter is facing a completely different problem," Zygmuntowicz, who noted that Engine Yard hosts 500 to 600 Ruby applications without scale problems.

On where the bottlenecks reside in Rails applications:

"The database is the bottleneck well before Rails," said Zygmuntowicz. Why? Rails features many modules that can eliminate a lot of database grunt work, but it only takes you 80 percent of the way there. The other 20 percent will require database coding. "Rails does insulate programmers from SQL but if you get too lazy with the object relationship you will get into trouble," said Zygmuntowicz. "As the site grows you will want to hand optimize the database. That's true for any language."

On the big benefit of Ruby on Rails:

Zygmuntowicz said the biggest benefit of Ruby on Rails is that "it will get you to market faster than any framework out there." The rub: That time to market will cost you less development time, but once it's in production it may require more servers. "That's a good tradeoff," said Zygmuntowicz. "Hardware is cheap and developers are expensive." I asked Zygmuntowicz what applications made the most sense for Ruby on Rails. He said Ruby shines for social networks and content presentation. It can also work for financial applications, but it can stumble if it has to "interface with a legacy database framework." "That can be a pain point," he added.

On his three Ruby on Rails lessons from his deployments:

"Ruby will take you 80 percent of the way there, but it is not an excuse for not having an actual architecture that will scale. That's the same for any other framework. You can't hope that the language will scale for you. Look at Friendster--it dropped out of the race because couldn't scale and they were in Java. Ruby on Rails makes everything so easy that you can get trapped into thinking it'll do everything for you. You still need to consider scalable Web architectures."

Divorce long running queries. Ruby on Rails typically serves one request at a time and that's a detriment if there are long running queries that block other incoming requests, said Zygmuntowicz. The workaround is that you have to set up a queuing mechanism so your database isn't overloaded.

You will have to do some database deep down coding. Ruby on Rails doesn't fix everything. In a staging environment everything will look fine, but once you go production you have to monitor the number of queries and the reaction time. "As you start to grow you will chew up some memory," said Zygmuntowicz. "The fix is to drop down into raw SQL coding to do the queries."

"Ruby on Rails is just as scalable as any other platform, but there's no free lunch," said Zygmuntowicz. "Ideally, Rails gets you there quicker and gives you time to think about how you scale."